I am with ZEN at home and have been for ~ 7 years. Of several ISPs I have used home and at work they are the only one with zero downtime and superb customer service. I recommend them highly.
On their old 512k/256k product they had no caps (and they still offer that I believe) but their current product (8192k/448k) is capped at 20GB each month. They email you when you reach 50% at or before the 15th.